Converted focal length; 360mm

[Ian Grant]

Ole, Schneider do claim that the Symmar's are Symmetrical, I think it was a War time Patent where Schneider first mooted an un-Symmetrical Symmar.

The two Schneider sites you posted links to post different data, but they don't contradict themselves. But they do claim the Symmar is a symmetrical lens.

The Symmar S series must be the plasmat series, according to what you are saying, which makes sense this is when they stopped being convertible.

Isn't the difference in focal lenght of the front & rear cells in a symmetrical pair used singly actually also to do with the position of the aperture.

Ian

[Ole]

The pre-WWII Symmars were Dagor-type f:6.8 (3 cemented elements in each cell), and were sold as triple convertibles. Post-WWII Symmars were Plasmats, (2 cemented and one air-spaced element in each cell), f:5.6, and still (!) unsymmetrical. The Symmar-S is also a plasmat, but the overall corrections have been improved by dropping the idea of the rear cell being moderately well-corrected alone (or so they claim).

If you put the front cell of a 240/420 Symmar convertible on the back of the shutter, you will not have a 420 f:12. You'll get something like a 500mm f:14. Don't confuse flange focal distance with focal length - I don't.

Symmetrical in lens design doesn't mean that the lens is absolutely symmetrical in the strict sense of the word, only that the two halves are of the same basic design. "Unsymmetrical" lenses are things like Tessars, where one cell is cemented and the other one not.

Fully symmetrical lenses with two cells of the same focal length can be found too - but that implies that they are optimised for 1:1 and not infinity. Repro and macro lenses are often fully symmetrical, with two identical cells.